A hydrostatic transmission (HST), in which a hydraulic pump and a hydraulic motor that are fluidly connected to each other and at least one of which is a variable volume type are respectively supported by a pump shaft and a motor shaft so as to be incapable of relative rotation and are accommodated in an HST case, is suitable for use in a drive-train power transmission path in utility vehicles such as riding lawn mowers and tractors (see Patent Document 1 below).
The above HST is useful in that it is capable of varying in a stepless manner the rotational speed of rotary power that is operatively input from the driving source into the pump shaft and outputting the resulting rotary power having a varied rotational speed from the motor shaft.
Meanwhile, for reducing the cost of a power transmission path, it is preferable to use general-purpose products for power transmission path components such as a driving source and an HST, but there are the following problems in such cases.
That is, when choosing a general-purpose HST, it is necessary to choose an HST having an allowable input rotational speed that is greater than the rated output rotational speed (the rated revolutions) of a driving source.
At that time, a general-purpose HST, the allowable input rotational speed of which is slightly greater than the rated output rotational speed of a driving source, is not always available, and, for example, there may be a case where a general-purpose HST, the allowable input rotational speed of which is 3500 rpm, has to be used with a driving source having a rated output rotational speed of 2600 rpm.
In such a case, only about 70% of the capacity of the HST is utilized, meaning that the performance of the HST is not fully utilized.
Here, in order to bring the speed of rotary power input into the pump shaft close to the allowable input rotational speed of the HST, it is conceivable to dispose between the driving source and the HST a speed-increasing gear mechanism including a drive shaft operatively connected to the driving source, a driven shaft disposed parallel to the drive shaft and operatively connected to the pump shaft, a drive-side gear supported by the drive shaft so as to be incapable of relative rotation, and a driven-side gear that is meshed with the drive-side gear, supported by the driven shaft so as to be incapable of relative rotation, and has a smaller diameter than the drive-side gear. This configuration, however, results in a power transmission path that is increased in size in the axial direction and the radial direction of the pump shaft.